Presumably, overheating problem.

So my PC randomly turns itself off as if it had overheated and I have not slightest idea why. Now yesterday I was playing graphic intense game and my PC turned itself off, I presumed that CPU or GPU had overheated and turned itself off for safety reasons, I was right because heatsink of my CPU was blocked by thick layer of dust, which I should have cleaned at least a month ago it was also rather hot, obviosly. So I cleaned the dust off heatsink and fans and because it was already late I went to sleep.

So today I turned on my PC logged into Win 7 desktop and after opening few Explorer windows my PC turned off, I tried it again and the same damn thing happened, I wanted to open tool to measure the temperature, but the damn PC shut off again.I thought it was OS problem and booted ubuntu, everything seemed to work fine for about 50 mintues and I was able to copy most of my important files on my external HDD.

Now I decided it was hardware problem so I went to BIOS few times and tried to find the damn place where temperature was shown, being illiterate computer ignoramus I wasn't able to find it and my PC shut off few times while I was in BIOS. The fans were working and the heatsink after my PC turned off in BIOS wasn't even lukewarm it was cool.

I honestly don't have any ideas what the problem is. Damaged CPU? Damaged temperature sensor? The heatsink wasn't moved, the fans were moving, dust was cleaned off and I was able to work in Ubuntu desktop for about hour without any problems or hiccups.

Hardware is

i7 clocked at 2.66Ghz
6 GB of ram
P6T motherboard
Nvidia GTS 250

Thanks for the help in advance.